A wind turbine nacelle of a wind turbine generator uses a large-diameter bearing. Since this bearing is expensive and the spare thereof is rarely prepared in advance, another bearing is manufactured in the event of being damaged, which takes half a year until the delivery. Accordingly, it is important to appropriately diagnose the state of degradation of a large-diameter bearing, particularly the main bearing of a wind turbine nacelle of a wind turbine generator, which is expensive and takes time to be manufactured, as early as possible.
Traditional diagnosis of degradation of a bearing of a wind turbine nacelle has been accomplished mainly by the vibration method. The vibration method detects displacement at a measuring portion of a machine element (e.g., bearing) with a sensor such as a strain gauge, detects the acceleration of the measuring portion, and detects the degree of degradation on the basis of a force acting on the measuring portion (see Patent Literature 1, for example).